Voltammetry, accessible timescales

UMEs decrease the effects of non-Earadaic currents and of the iR drop. At usual timescales, diffusional transport becomes stationary after short settling times, and the enhanced mass transport leads to a decrease of reaction effects. On the other hand, in voltammetry very high scan rates (i up to 10 Vs ) become accessible, which is important for the study of very fast chemical steps. For organic reactions, minimization of the iR drop is of practical value and highly nonpolar solvents (e.g. benzene or hexane [8]) have been used with low or vanishing concentrations of supporting electrolyte. In scanning electrochemical microscopy (SECM [70]), the small size of UMEs is exploited to locahze electrode processes in the gm scale. 

When diffusion layers overlap by a large amount, an overall planar response will be expected, but with a characteristic area equivalent to the total array surface area rather than just the electroactive surface area. Hence, the Case 4 current will be (1/ ) times larger than the Case 1 current. This will occur when X(jiff d where d is the separation of the individual microdiscs. Therefore, Case 4 behaviour arises at t 0.1 s. This will therefore be the dominant behaviour for cyclic voltammetry at normal scan rates at this particular array. With chronoamperometry, short timescales are accessible and so Case 3 behaviour may also be observed. 

As with microelectrodes, diffusive transport to nanoelectrodes on conventional voltammetric timescales is dominated by convergent, as opposed to planar, diffusion. Therefore, for a simple electron transfer process, the voltammetric response at steady state is characterised by a sigmoidal shape. Simulation of such voltammetry requires solution of the diffusion equation typically with a Nemstian or Butler-Vofiner boundary condition for the rate of electron transfer at the electrode surface, depending on its reversibility. For simple, uniformly accessible, electrode geometries analytical solutions of these equations are available, and so for a disk electrode we obtain the familiar equation for the current (iiim) in the limit of diffusion control ... 

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